The objective of the project is to characterize the reactions of sulfido ligands in electron rich molybdenum-sulfur and molybdenum-iron-sulfur complexes. Previous studies of related neutral molybdenum dimers have established that the sulfido ligands are active sites in the hydrogenation of alkynes, azocompounds and other unsaturated molecules. The proposed studies will extend this ligand based reactivity in several ways so that it more closely models the chemistry of nitrogenase. Anionic derivatives with negative reduction potentials will be synthesized by means of chemical and electrochemical reductions. Steichiometric reactions of the sulfido ligands in these complexes with protons and with unsaturated molecules in the presence of protons will be studied. Intermediates will be characterized when possible and final reduction products will be identified. The catalytic reduction activity of electrochemically generated derivatives will be assessed. The goal of the project is to identify new ligand based reduction mechanisms and to investigate their relevance to the chemistry of nitrogenase. The studies of these model systems will contribute to the long term goal of understanding the mechanism of the enzyme.